The present invention relates to a transmission system for determining a position of a mobile station by using communication satellites.
Conventionally, a position of a mobile station, which incorporated in a vehicle such as an automobile or a ship or an aircraft, is determined by using communication satellites. In the case of using three or more communication satellites, signals from these satellites are received by the mobile station. Then, on the basis of information obtained from receiving timings, a linear equation with four unknowns is solved to thereby calculate the position of the mobile station. This position determining system requires many satellites, and the mobile station requires to be equipped with a number of receivers corresponding to these satellites. Furthermore, it is necessary that precise computation for the position determination is to be carried out in the mobile station.
In contrast, there has been proposed in U.S. Pat. No. 4,839,656, for example, a position determining system employing two geostationary satellites and carrying out the computation for the position determination in a fixed station on the ground.
This known position determining system is constructed as illustrated in FIG. 3, for example. Referring to FIG. 3, reference numeral 1 designates a mobile station such as a truck whose present position is intended to be determined. The mobile station 1 is provided with a receiver for receiving a radio wave from a first geostationary satellite 2, a first transmitter for transmitting a radio wave to the first geostationary satellite 2, and a second transmitter for transmitting a radio wave to a second geostationary satellite 3. On the other hand, a fixed station 4 on the ground is provided with a transmitter for transmitting a radio wave to the first geostationary satellite 2, a first receiver for receiving a radio wave from the first geostationary satellite 2, and a second receiver for receiving a radio wave from the second geostationary satellite 3. Additionally, another fixed station 5 for calibrating the position of the mobile station 1 is located at a position remote from the fixed station 4. The position calibrating fixed station 5 is provided with a receiver for receiving a radio wave from the first geostationary satellite 2, a first transmitter for transmitting a radio wave to the first geostationary satellite 2, and a second transmitter for transmitting a radio wave to the second geostationary satellite 3.
Next, a procedure for determining a position of the mobile station 1 will be described with reference to FIG. 4. First, a synchronizing signal precisely managed in time is transmitted from the fixed station 4 to the first geostationary satellite 2. This synchronizing signal is relayed by the first geostationary satellite 2, and is then received by the receiver mounted on the mobile station 1. In regard to the transmission of the synchronizing signal from the fixed station 4 through the first geostationary satellite 2 to the mobile station 1, reference character t.sub.1 denotes a time required for transmitting the synchronizing signal to the fixed station 4 to the first geostationary satellite 2, and reference character t.sub.2 denotes a time required for transmitting the synchronizing signal from the first geostationary satellite 2 to the mobile station 1.
After a predetermined time t.sub.0 has elapsed from a receiving timing of this synchronizing signal at the mobile station 1, a first packet signal containing an ID number of a terminal in the mobile station 1 and also containing information contained in an input signal is transmitted from the mobile station 1 to the first geostationary satellite 2. At the same time, i.e., after the predetermined time t.sub.0 has elapsed from the receiving timing of this synchronizing signal, a second packet signal similar to the first packet signal is transmitted from the mobile station 1 to the second geostationary satellite 3. The above predetermined time t.sub.0 is a constant value, and information of this time t.sub.0 is preliminarily stored in the fixed station 4. In regard to the transmission of these packet signals from the mobile station 1 to the first and second geostationary satellites 2 and 3, reference character t.sub.2 ' denotes a time required for transmitting the first packet signal from the mobile station 1 to the first geostationary satellite 2, and reference character t.sub.3 denotes a time required for transmitting the second packet signal from the mobile station 1 to the second geostationary satellite 3.
The first and second packet signals are relayed by the first geostationary satellite 2 and the second geostationary satellite 3, respectively, and are then received by the fixed station 4. In regard to the transmission of these packet signals from the first and second geostationary satellites 2 and 3 to the fixed station 4, reference character t.sub.1 ' denotes a time required for transmitting the first packet signal from the first geostationary satellite 2 to the fixed station 4, and reference character t.sub.4 denotes a time required for transmitting the second packet signal from the second geostationary satellite 3 to the fixed station 4.
Thereafter, in the fixed station 4, a distance between the first geostationary satellite 2 and the mobile station 1 is calculated from a receiving timing of the first packet signal received from the first geostationary satellite 2, a transmitting timing of the synchronizing signal transmitted from the fixed station 4, and a distance between the fixed station 4 and the first geostationary satellite 2. Similarly, a distance between the second geostationary satellite 3 and the mobile station 1 is calculated from a receiving timing of the second packet signal received from the second geostationary satellite 3, the transmitting timing of the synchronizing signal, and a distance between the fixed station 4 and the second geostationary satellite 3. More specifically, the distance between the fixed station 4 and the first geostationary satellite 2 is a fixed value, and the distance between the fixed station 4 and the second geostationary satellite 3 is also a fixed value. Therefore, these distances can be preliminarily determined in the fixed station 4. Accordingly, the transmission time t.sub.1 and the transmission time t.sub.1 ' between the fixed station 4 and the first geostationary satellite 2 can be determined from the above fixed distance between the fixed station 4 and the first geostationary satellite 2. Similarly, the transmission time t.sub.4 between the fixed station 4 and the second geostationary satellite 3 can be determined from the above fixed distance between the fixed station 4 and the second geostationary satellite 3. In this case, the time t.sub.1 is equal to the time t.sub.1 ' because the distance between the fixed station 4 and the first geostationary satellite 2 is fixed. The remaining transmission times t.sub.2, t.sub.2 ' and t.sub.3 varies with a position of the mobile station 1. As to the transmission times t.sub.2 and t.sub.2 ', since the transmission between the mobile station 1 and the first geostationary satellite 2 is carried out in the same distance, the transmission times t.sub.2 and t.sub.2 ' are equal to each other. Accordingly, the transmission time t.sub.2 (=t.sub.2 ') can be calculated by subtracting the known times t.sub.1, t.sub.1 ' and t.sub.0 from a total time t.sub.a required for the transmission from the transmitting timing of the synchronizing signal to the receiving timing of the packet signal via the first geostationary satellite 2. After thus determining the transmission time t.sub.2, the transmission time t.sub.3 can be calculated by subtracting the known times t.sub.1, t.sub.2, t.sub.4 and t.sub.0 from a total time t.sub.b required for the transmission from the transmitting timing of the synchronizing signal to the receiving timing of the second packet signal via the second geostationary satellite 3.
After thus calculating these transmission times t.sub.2 and t.sub.3, information of the times t.sub.2 and t.sub.3 is converted into information of distances on the basis of a transmission rate, thus obtaining the distance between the mobile station 1 and the first geostationary satellite 2 and the distance between the mobile station 1 and the second geostationary satellite 3. Then, on the basis of these two distances and information of precise positions of the first and second geostationary satellites 2 and 3, a two-dimensional position of the mobile station 1 is calculated in the fixed station 4.
Then, by using this two-dimensional position calculated above and a data base of a terrain map provided in the fixed station 4, a three-dimensional position of the mobile station 1 is calculated.
A manner of calculating the three-dimensional position of the mobile station 1 in the fixed station 4 will now be described with reference to FIG. 5. Referring to FIG. 5, reference character dl denotes a distance between the mobile station 1 and the first geostationary satellite 2 which exists on a predetermined orbit, and reference character d.sub.2 denotes a distance between the mobile station 1 and the second geostationary satellite 3 which also exists on the above predetermined orbit. A point on the earth E remote from the first geostationary satellite 2 by the distance d.sub.1 describes a circle c.sub.1, and a point on the earth E remote from the second geostationary satellite 3 by the distance d.sub.2 describes a circle c.sub.2. Accordingly, two intersections e.sub.1 between the circles c.sub.1 and c.sub.2 exist on the earth E, one of which intersections e.sub.1 existing on the northern hemisphere, while the other existing on the southern hemisphere. A coordinate position of each intersection e.sub.1 can be determined from the data base of the terrain map.
Further, in carrying out the calculation of the coordinate position of the mobile station 1, the transmission of signals may be carried out between the fixed station 4 and the position calibrating fixed station 5 in each service area via the first and second geostationary satellites 2 and 3, so as to calibrate the above calculated coordinate position according to the signal returned to the position calibrating fixed station 5.
In the above prior art transmission system for determining a position of the mobile station 1, it is necessary to carry out the two-line inbound transmission from the mobile station 1 to the fixed station 4 and the one-line outbound transmission from the fixed station 4 to the mobile station 1. That is, the mobile station 1 needs to have two sets of transmitters consisting of the first transmitter for transmitting a radio wave to the first geostationary satellite 2 and the second transmitter for transmitting a radio wave to the second geostationary satellite 3. In general, such a transmitter for transmitting a radio wave to a geostationary satellite requires a relatively large transmission antenna, for example. Accordingly, it is not easy to mount the two sets of transmitters on a relatively small mobile station such as an automobile. Further, this prior art transmission system requires two geostationary satellites for relaying relatively small power radio waves from the mobile station, causing an increase in cost for the construction of the system.